Aluminum etchant compositions



'fine surface appearance.

United States Patent O 3,083,165 ALUMINUM ETCHANT COMPOSITIONS James W. Carroll, Lafayette Hill, Pa., assignor to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania N Drawing. Filed Sept. 10, 1958, Ser. No. 760,095 7 Claims. (Cl. 252-795) This invention relates to compositions for the deep etching of aluminum and aluminum alloys by contacting the metals with aqueous solutions of alkali metal hydroxides modified with inorganic and organic additives. The compositions of the invention comprise mixtures of the alkali metal hydroxides with the additives in dry form which upon solution in water deep etch aluminum and aluminum alloy surfaces. These compositions leave the aluminum surfaces in a smooth even condition from which smut deposits are easily removed.

By deep etching of aluminum and aluminum alloys is meant the removal of substantial amounts of the metal by chemical action. Deep etching replaces mechanical machining operations on aluminum and aluminum alloys in many applications. This process of deep etching has come to be known as chemical milling or contour etching or profile etching. Deep etching permits the milling of complex shapes and relatively sharp corners, all of which may be processed in one or more etching operations. The deep etching process is not limited as to the directions of cut or the radius of cutters as is the case with standard machining operations.

Etching also permits the design of lighter weight structures which are particularly useful in the aircraft industry. Deep etching of aluminum has advantages, in addition to the complex shapes that can be made, in that it provides for the formation of extensive shape modifications by unskilled operators in an assembly line operation. In general, the processes are so set up that they are handled on a continuous basis in a dip tank assembly line. Deep etching also has the economic advantage that the parts after being etched to the proper dimensions do not need further smoothing or polishing as is conventional with mechanical matching operations.

The physical and chemical properties of the etched aluminum or aluminum alloy metal part are essentially the same as an unetched part which has been machined and polished. For most engineering purposes there is no great distinction between deep etched and machine milled aluminum or aluminum alloys. However, there may be considerable variations in the surface appearance of the aluminum depending on the modification of the etching solution used.

Satisfactory deep etching of aluminum requires that the surface of the metal after is has been etched be smooth and uniform. By smoothness of surface is meant the fineness of the residual grain structure with will give a By uniformity of surface is meant the evenness of the metal removal for all parts of the metal subject to the chemical attack. Solutions which do not give a uniform etching will leave ridges and pits in the metal which, may vary by as much as 10 mils difference from part to part.

Integral with the problem of smoothness of surface and evenness of metal removal is the problem of smut removal. Smut is the residual metal particles which adhere to the aluminum or alloy surface after the major portion of the metal has been dissolved by the modified etching solution. Satisfactory etching processes must leave this smut in a loosened condition so that it can be readily and easily removed by an acid rinse following the etching solution.

The actual removal of the aluminum metal is effected 3,083,165 Patented Mar. 2 6, 1 9 63 by chemical reaction with a caustic alkali such as sodium hydroxide, potassium hydroxide and other alkalinematerials. Use of the caustic materials alone results in the production of an unsmooth surface, a surface with may vary in uniformity and a surface which may also be encrusted with smut which cannot thereafter be readily re;- moved. Accordingly, the caustic solutions used for deep etching must be modified by suitable chemical agents in order that the etched metal will have the desired smoothf ness, uniformity and be readily desmutted.

It is the principal purpose of my new compositions when dissolved in water to provide modified alkali metal hydroxide etching solutions which will leave the aluminum surface in a smooth, even condition and from which smut deposits are easily removed after chemical milling.

I have discovered that aluminum etchant compositions comprising alkali metal hydroxides and one or more members selected from the group consisting of sodium butyrate, sodium allyl sulfonate, sorbitol hydroxyethylether and sorbitol hydroxypropylether in aqueous solution leave the aluminum or aluminum alloy surface in a smooth and even condition. The above compositions are prepared by adding the organic materials to the caustic soda. In the case of sodium butyrate and sodium allyl sulfonate which are powders the materals are blended together with the dry anhydrouscaustic soda. While the sorbitol hydroxyethylether and sorbitol hydroxpropylether have been described, it is also known that other sorbitol hydroxyalkylethers where the alkyl group does not exceed 8 carbon atoms are useful in my compositions 'and processes.

The sorbitol hydroxyether and the sorbitol hydroxypropylether may be convenientlypurchased under the trade names Atlas G2240 and Atlas G240l, respectively, from the Atlas Powder Company. The organic materials are used at from /2 to 5% based on the weight of the caustic soda. Additives at a ratio of less than /2% are generally ineffective in producing smoothness of surface and evenness of metal removal. Additions greater than 5% of the caustic concentration do not usually produce additional surface benefits to the metal and are not recommended.

In the case of the sorbitol hydroxyethylether and sorbitol hydroxypropylether, which are liquids, these materials are sprayed or otherwise mixed with the caustic soda which readily absorbs them when added in the amount of /2 to 5%, and the resulting mixture maintains the physical condition and appearance of the caustic soda alone.

These compositions are used in water at a caustic soda concentration of from 8 to 25% by weight. Generally, the temperature of the etching bath is maintained within the range of to 210 F. Selective deep etching 'action is obtained by masking off part of the area which it is not desired to etch and then exposing the unmasked portion of the aluminum or aluminum alloy to the action of the etchant bath. The amount of metal removed'will be found to be proportional to the time of the exposur of the metal to the etchant material.

The above compositions comprising caustic soda and a member selectedfrom the group consisting of sodium butyrate, sodium allyl sulfonate, sorbitol hydroxyethylether and sorbitol hydroxypropylether represent compositions in which either one of the organic materials is added to the caustic or they may be compositions in which more than one of the agents found to promote smoothness of etching are combined and added to the caustic soda. Where more than one smoothing additive is mixed with the caustic soda, the total concentration need be no more than if only one were used, namely, in the range of /2 to 5%.

The above compositions have been found to be extremely effective in promoting smoothness and evenness of etching on aluminum and certain aluminum alloys and are particularly effective in deep etching 14S and 24S, aluminum alloys which are commonly used in the aircraft and other industries.

The advantages of these compositions in etching aluminum can be observed by studying Table I. Table I represents chemical milling experiments which were carried out on aluminum alloy as plates or panels. The panels were suspended in the test solution vertically. The concentration of the caustic soda in the aluminum etchant bath was adjusted between 12 to 24 Ounces per gallon, and the temperature was maintained at between 180 to 220 F. When small test panels were being observed in laboratory size containers, there was no need to agitate the solution but in larger size tests where large tanks were used, it was found desirable to agitate the bath by an air current or other means. The test specimens were uniformly exposed to the etchant bath for a period of 1 hour. In this 1 hour period it was usual to expect removal of 60 mils of aluminum or about 1 mil per minute under the conditions of concentration and temperature specified. It is important in making comparative tests of etchants on alloys that the same amount of aluminum in each case be removed since it is well known in the art that smoothness of metal surfaces tends to decrease as the amount of aluminum removed increases.

TABLE I Effect of Additives to Caustic Soda on Chemical Milling or Aluminum Alloy 24S In Table I and in the other tables through the specification the surface condition was generally obtained by the use of a Brush profilometer which gives the smoothness of the surface in microinches (millions of an inch). In chemical milling work readings of 100 microinches or below are considered good, that is, quite smooth, while readings below 200 are acceptable. Thus, a surface condition described as smooth reflects a profilometer reading of 100 or less while slightly rough represents a profilometer of 100 to 200, while roug represents a reading in excess of 200.

The test data (above) confirm that sodium allyl sulfonate, sodium butyrate, sorbitol hydroxypropylether and sorbitol hydroxyethylether are particularly useful in treating type 245 alloys. This alloy was chosen for the smoothness tests since it is the alloy on which is most difficult to obtain smooth surfaces.

I am also aware of the work of Newman et al. in this same field of aluminum etching as disclosed in United States Patents 2,795,490 and 2,795,491, which patents issued June 11, 1957. These patents teach that elemental sulfur, alkali metal sulfides and polysulfides, sodium thiosulfate and certain organic sulfides containing reactive sulfur assist in the promotion of a non-nodulating surface when used with a caustic etching material.

I have now discovered that when the alkali metal sulfide materials found useful by Newman are combined with the compositions which I have discovered that further unexpected improvements in deep etching of aluminum and aluminum alloys can be obtained. The resulting aluminum surfaces are more uniform and of a much smoother appearance than if the alkali metal sulfides of Newman were used alone or if the above compositions disclosed by myself are used alone. In other words the smoothening and evening effect of the alkali metal sulfide combined with the materials found active by myself are synergistic and are of greater etficiency than when either type is used alone.

These additivesboth the organic material and the alkali metal sulfideare each used in the range of /2 to 5% of the weight of the alkali metal hydroxide. In general, the concentrations, temperatures and time of immersion in the etchant bath are the same as for the materials used in Table I. The elficiency of these compositions in treating aluminum alloy 248 can be observed in Table II.

In these tests the organic additive was combined with the caustic sode at 2% of the caustic soda weight. Sodium sulfide was also present at 2% of the caustic soda weight. The test panels used in Table II were aluminum alloy 248. The etchant concentration in the bath was 24 ounces per gallon, and the samples were suspended vertically in the bath at 180 F. for a period of 1 hour. After the etching period, the samples were removed, rinsed, washed in chromic acid, dried and then measured with the profilometer. The observations of the surface of the metal after etching are coded the same as those in Table I.

TABLE II Efiect of Organic Additives and Alkali Metal Sulfide an Aluminum Etching [Concentration of etchant-24 ounces per gallon; concentration of additive-each 2% of caustic; Temperature-lF.]

The tests in Table II show that the compositions comprising alkali metal sulfide and one or more members of the group consisting of sodium butyrate, sodium allyl sulfonate, sorbitol hydroxyethylether and sorbitol hydroxypropylether when combined with caustic soda are particularly useful on aluminum alloys such as 24S. The data also show that the combination of additives is superior to the use of sodium sulfide alone.

However, even these improved solutions combining sodium sulfide and the smoothening agents found useful by me are not completely satisfactory when deep etching particular types of alloys such as 758, even through the surface produced is smooth. The particular difiiculty arises in handling the smut which accumulates in particular alloys represented by 758. The smut remains as an adherent material which is not readily removed by the action of the chromic acid rinse or other acid rinses normally used for desmutting operations. This smut build-up is believed to be caused by residual amounts of copper, magnesium, silicon and zinc metal left in the form of metal solids on the surface of the etched material and which builds up progressively as the alloy is milled.

I have observed that the aluminum etchants represented by sodium allyl sulfonate, sodium butyrate, and the sorbitol hydroxyethers whether used alone with alkali metal hydroxide or in combination with alkali metal sulfide and hydroxide are only partially effective in leaving the smut in a condition which is readily removed from such alloys as 758. Accordingly, it is a further aspect of my invention that I have found compositions which are effective in aiding smut removal and which will at the same time produce a smooth etch and a uniform etch which is applicable to any aluminum or aluminum alloy compositions.

I have now discovered that compositions comprising alkali metal hydroxide, sugar, alkali metal sulfide and one or more of the member of the group consisting of sodium butyrate, sodium allyl sulfonate, sorbitol hydroxyethylether and sorbitol hydroxypropylether when present in certain ratios will chemically mill aluminum and aluminum based alloys and produce a smooth even etch while at the same time leave the smut in an easily removable condition. Composition employing these materials are particularly useful in the following ranges: alkali metal hydroxide-80 to 97%, sugar-2 to alkali metal sulfide-V2 to 5% and one or more members of the group specified above /2 to 5%.

0f the alkali metal hydroxides and sulfides which are useful in my compositions, the sodium salts are most useful because they are the least expensive. Similarly, sucrose is the most available and least expensive of the sugars. While sodium butyrate, sodium allyl sulfonate, soribtol hydroxyethylether and sorbitol hydroxypropylether are all useful in the above combination to promote removal of smut. Compositions employing the above materials are particularly useful in the following ranges:

Examples showing the improved benefits obtainable using these compositions in treating aluminum alloy 758 are presented in Table III.

Alloy 758 was selected as the test alloy because the smut residual on deep etching is only removed with great ditficulty. This is possibly due to the high concentration of zinc.

Tests to compare the effectiveness of known deep-etching additives with the novel compositions of my invention are shown in Table III. In this group of runs etchant compositions comprising sodium hydroxide with one percent of sodium sulfide were compared with the same material to which two parts by weight of sorbitol hydroxyethylether on alloys 24S and 758 were added at two different time periods. The evenness measurements in microinches were made with the Brush profilometer, both with the metal grain and across the metal grain.

TABLE III Comparative Deep Etching at and 80 Minute Periods [NaOH c0nc.11.2 ounces/gallon and 190 F. temp] The above tests demonstrate the eifectiveness of the sorbitol hydroxyethylether in promoting evenness of etching. In general, the sorbitol hydroxyethylether improved the evenness of the metal surface as compared with deep etching using the sulfide additive. 0n the tests at the 80 minute period where the surface evenness using the sulfide additive alone appeared as good as the compositions consisting of sodium hydroxide, sodium sulfide and sorbitol hydroxyethylether, it must be observed that in the case of the latter, mils of aluminum were removed as compared to only 67.5 with the former. Thus, 33%

more aluminum was removed where the sorbitol ether was employed.

All of the aluminum etchant compositions, as above disclosed by me, are used with caustic alkali solutions generally in the order of 8 to 25% sodium hydroxide. The concentration of the sodium hydroxide is not critical, and it may be slightly higher or slightly lower in any particular case. It is generally not advisable to go below 8% caustic concentration in order to prevent any formation of aluminum scale of the type which is ordinarily encountered in the surface etching processes.

Similarly, the temperature at which the etching operations are performed may vary within the range of to 210 F., and again the temperature is not critical. As was discussed above, the time of immersion in the alkali etchant bath will determine the amount of metal removed which is in contact with the caustic alkali. The longer the material is in contact with the caustic, the more aluminum there will be removed. Generally, the amount of aluminum removed by the caustic alkali will be directly proportional to the time of exposure to the caustic.

As discussed above, the concentration of the additives of the various compositions is in the range indicated to be found useful by me. The range of concentrations disclosed shows the economic usefulness of the compositions. Additives below /z% would generally not be effective in producing the smoothness, evenness of etching and ready removal of smut. Amounts greater than 5% may be used, but these additional amounts usually do not produce additional smoothness or additional evenness of etching or additional removal of smut.

I cannot account for the theory to explain the unusual metal surface modifications produced by my compositions. I only know that these particular materials when combined and used in the particular manner set forth in the specification and claims produce the unusual results encountered when treating aluminum and aluminum alloys. When these materials are not combined in the manner found useful by me, considerably poorer results are obtained. In many instances complete lack of smoothness and complete lack of uniformity of etching are encountered.

I have found certain additives which are useful in producing smoothness and uniformity in deep etching of aluminum, but they are generally unsatisfactory because of other reasons. For example, sodium naphthalene sulfonate will contribute some smoothening and evening action in treating aluminum but is an undesirable additive because of the excessive foaming produced. Other material-s which leave a white deposit or are generally unsatisfactory for the promotion of smooth and even etching and for the ready removal of smut deposits are propionic acid, benzoic acid, glucamine, caproic acid, crotonic acid, naph thenic acid, sebacic acid and adipic acid.

It wil be understood, that while this invention has been described with reference to certain; preferred embodiments thereof, various modifications and changes may be made therein within the spirit of the invention. Unless otherwise stated, the percentage figures appearing in the claims refer to percent by weight.

I claim:

1. An aluminum etchant composition consisting essentially of alkali metal hydroxide in admixture with the following additives expressed as a percentage by weight of the said alkali metal hydroxide:

/2 to 5% of alkali metal sulfide, and,

' /2 to 5% of at least one member selected from the group consisting of sodium butyrate and sodium allyl sulfonate.

2. An aluminum etchant composition consisting essentially of caustic soda in admixture with the following additives expressed as a-percentage by weight of the said caustic soda:

/z to sodium sulfide, and,

/2 to 5% of at least one member selected from the group consisting of sodium butyrate and sodium allyl sulfonate.

3. An aluminum etchant composition consisting essentially of caustic soda in admixture with /2 to 5% by Weight of the said caustic soda of at least one member selected from the group consisting of sodium butyrate and sodium allyl sulfonate.

4. An aluminum etchant composition consisting es sentially of alkali metal hydroxide in admixture with the following additives expressed as a percentage by weight of the said alkali metal hydroxide content:

/2 to 5% alkali metal sulfide, 2 to sugar, and,

/2 to 5% of at least one member selected from the group consisting of sodium butyrate and sodium allyl sulfonate.

5. A bath for chemical milling aluminum consisting essentially of the composition of claim 1 dissolved in water at a concentration range of 8 to by weight alkali metal hydroxide.

6. A bath for chemical milling aluminum consisting essentially of the composition of claim 2 dissolved in Water at a concentration range of 8 to 25% by weight caustic soda.

7. A bath for chemical milling aluminum consisting essentially of the composition of claim 4 dissolved in water at a concentration range of 8 to 25 by weight alkali metal hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,795,491 Newman et al June 11, 1957 2,853,372 McDonald et a1 Sept. 23, 1958 

1. AN ALUMINUM ETCHANT COMPOSITION CONSISTING ESSENTIALLY OF ALKALI METAL HYDROXIDE IN ADMIXTURE WITH THE FOLLOWING ADDITIVES EXPRESSED AS A PERCENTAGE BY WEIGHT OF THE SAID ALKALI METAL HYDROXIDE: 1/2 TO 5% OF ALKALI METAL SULFIDE, AND, 1/2 TO 5% OF AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF SODIUM BUTYRATE AND SODIUM ALLYL SULFONATE. 